1. Field of the Invention
This invention relates to the polymerization of olefins and more particularly relates to catalyst compositions useful for polymerizing one or more monomers comprising ethylene to polymers having an intermediate molecular weight distribution and a good balance of physical properties.
2. Description of the Prior Art
It is known that catalysts of the type variously described as coordination, Ziegler, Ziegler-type, or Ziegler-Natta catalysts are useful for the polymerization of olefins under moderate conditions of temperature and pressure. It is also known that the properties of the polymers obtainable by the use of such catalysts, as well as the relative economies of the processes used to prepare the polymers, vary with several factors, including the choice of the particular monomers, catalyst components, polymerization adjuvants, and other polymerization conditions employed.
During the years since Ziegler catalysts were first publicly disclosed, there has been a considerable amount of research conducted on the use of such catalysts; and numerous publications have resulted from that research. These publications have added much to the knowledge of how to make various types of olefin polymers by various types of processes. However, as is apparent from the amount of research on Ziegler catalysis that is still being conducted throughout the world, as well as the number of patents that are still being issued to inventors working in the field of Ziegler catalysis, the means of attaining certain results when polymerizing olefins with Ziegler catalysts are still frequently unpredictable. The fact that this situation exists in sometimes due to the need to obtain a previously-unattainable combination of results; occasionally due to difficulties in obtaining the same results in a commercial-scale apparatus as in a laboratory-scale reactor; and often due to a polymerization parameter's having an effect, or side-effect, in a given type of polymerization process that is different from effects achieved by its use in prior art processes of a different type.
One aspect of Ziegeler catalysis in which the need for further research has been found to exist has been in the field of preparing ethylene polymers having an intermediate molecular weight distribution and a good balance of physical properties. Such polymers have particular application in the production of sheets and certain articles that are formed by blow molding, e.g., containers for milk, and--like other polymers intended for commercial use--are desirably prepared by a process which is as economical as possible as well as being capable of producing a polymer having the desired properties.
There are, of course, known processes for preparing resins having an intermediate molecular weight distribution by polymerizing ethylene with the aid of Ziegler catalysts. However, the known processes typically suffer one or more of the disadvantages of lack of economy, inability to produce polymers having a suitable balance of properties, and/or unreliability in producing such polymers--particularly in commercial-scale operations. The processes described in Monsanto's British Pat. No. 1,489,410 are among such processes. Actually, from the teachings of the patent and what can be learned from repeating its examples, it is apparent that Monsanto's processes were designed for the preparation of blow-molding resins having broader molecular weight distributions than are desired for many blow-molding resins. However, with the use of technology not taught by Monsanto, the processes can sometimes be manipulated to produce resins having intermediate molecular weight distributions as well, and they appear to be better than many known processes for preparing blow-molding ethylene polymers in some respects, e.g., in their use of supported Ziegler catalysts having a vanadium component and in their ability to produce polymers having desirable properties in laboratory-scale operations. It has been found, however, that they are unreliable in producing polymers having intermediate molecular weight distributions.
Attempts to make Monsanto's processes more reliable in producing polymers having suitable and predictable intermediate molecular weight distributions have heretofore been unsuccessful. Polymers having a variety of molecular weight distributions ranging from broad to narrow have been produced by modifying Monsanto's teachings in various ways, but a "tunable" variable, i.e., a component that can be varied in a substantially linear way so as to permit the production of polymers having a predictable variety of intermediate molecular weight distributions has not previously been discovered.
What is still needed is a catalyst which (a) is suitable for use in a gas-phase polymerization process, (b) has sufficient activity to be economically attractive, and (c) contains a tunable variable that permits the production of ethylene polymers having predictable intermediate molecular weight distributions and a good balance of physical properties.